Vettel, only months removed from wrapping up his most dominant season in F1, was instructed to allow debutant Red Bull teammate Daniel Ricciardo to pass him midway through Sunday's Chinese Grand Prix as the German's speed began to fade due to his rapidly degrading tires.

Vettel initially fought off Ricciardo's attempts to get by and testily asked his team which tires the Australian was using. When told both drivers were on medium tires, Vettel responded over the radio, "Tough luck."

He did finally accede, but his initially defiant response raised questions after the race whether there was discord simmering in the Red Bull camp. The incident came just two weeks after Vettel was ordered by the team to let the quicker Ricciardo pass when they were fighting for position in the Bahrain GP.

Team principal Christian Horner tried to defuse the situation after Sunday's race by explaining the team had different strategies for the drivers given the varying wear of their tires — the team intended to have Ricciardo stop just twice and Vettel three times.

"They race for a team at the end of the day. They both understand that. Our objective is to maximize our chances and our best chance in the race today was with Daniel," Horner said.

Vettel also downplayed the incident, saying he moved aside as soon as he understood the strategies. But he still struck a downcast note as he struggles to adjust to the team's newly redesigned car with the turbo V6 hybrid engine introduced to the sport this season.